1. Field of the Invention
The present invention relates to an analog/digital circuit.
2. Description of the Related Art
In recent years, an FM radio station in Europe and a radio broadcast station in U.S. transmit additional information such as text as information in an RDS (Radio Data System) in Europe or RBDS (Radio Broadcast Data System) in U.S. By using the RDS/RBDS, in a receiver such as a car radio, various information such as the name of a broadcast station which is presently tuned, genre of music, and the like can be used (Japanese Patent Application (Laid Open) Nos. H8-256135 and H8-191232).
On the other hand, an FM transmitter for converting an audio signal to a stereo composite signal, performing frequency modulation by using a frequency modulator, and outputting the frequency modulated signal is known (Japanese Patent Application (Laid Open) Nos. H9-069729, H10-013370, and H9-312588). By using the FM transmitter, the audio signal can be transmitted without using a wire such as an RCA cable. Consequently, the FM transmitter can be used for transmitting signals between a CD changer in a car audio system and a head unit. Moreover, in recent years, a hard disk audio device, a memory audio device, and a cellular phone terminal having the music reproduction function are markedly being spread. The FM transmitter is used also for the application of reproducing music data stored in such a small electronic device from a speaker of a stationary audio component or the like.
Generally, the FM transmitter includes a filter unit, a stereo modulator, and a frequency modulator. The filter unit includes a pre-emphasis filter and a low-pass filter. The stereo modulator performs stereo-modulation on an output of the filter unit, thereby generating a stereo composite signal. The frequency modulator modulates the frequency of a carrier wave using the stereo composite signal as a modulation signal.    [Non-patent document 1] National Radio Systems Committee, “United States RBDS Standard”, U.S., Apr. 9, 1998
A study will be made on the case of providing a digital-to-analog converter (hereinbelow, simply called DA converter) between a stereo modulator and a frequency modulator, performing a stereo modulation process in a digital manner, and performing frequency modulation in an analog manner.
Generally, the operation clock of the filter unit and that in the stereo modulator for performing the digital process are different from each other. The operation clock of the stereo modulator and that of an analog-digital converter are also different from each other. In such a situation, it is an important issue to determine a frequency at which each of the circuit blocks operates from the viewpoint of improving circuit characteristics such as the S/N ratio and distortion.
Such a problem may occur not only in the FM transmitter but also in a analog/digital hybrid circuit in which both of a digital circuit and an analog circuit are integrated.